Spina bifida is a relatively common, structural malformation that is associated with excess morbidity and mortality. A specific etiologic agent(s) cannot be identified in the majority of individuals with spina bifida, and in this group of patients the condition is believed to be a genetically complex trait. As with other complex human traits, spina bifida is thought to be influenced by common genetic variants that, individually, may have only a small to moderate effect on risk. However, efforts to identify such variants have been hampered by lack of replication. This is, at least in part, attributable to overly simplistic models of disease etiology, poor experimental design and insufficient sample size. The studies proposed in this application are designed to minimize these limitations while addressing the primary study hypothesis: common variants in folate-related genes contribute to the risk of spina bifida. Our evaluation of this hypothesis will consider the roles played by both the maternal and embryonic genotype, and the possibility that the interplay of genes and environmental risk factors may be more relevant to the risk of spina bifida than is the independent main effect of any one susceptibility locus. This twice revised, competitive renewal application builds on our previous work by: (1) extending our collection of families, (2) increasing the number of folate-related genes to be evaluated, and moving from a single-SNP to a multiple-SNP per gene approach, (3) incorporating new methods for the assessmentof complex interactions, and (4) extending our recently developed, likelihood- based approach for evaluation of maternal and embryonic genetic effects to allow for missing data, genotyping errors and both gene-environment and gene-gene interactions. These studies will help to define the relationship between spina bifida risk and variation in folate-related genes. Such an understanding will provide improved content for genetic counseling, including the possibility of genotype-based, risk- assessment, and the foundation for novel new prevention strategies for this common, serious malformation.